Many modern aircraft are equipped with an airborne auxiliary power unit (“APU”) that generates and provides electrical and pneumatic power to various parts of the aircraft for such tasks as environmental control, lighting, powering electronics, and main engine starting. It is known that cooling the APU oil and engine externals increases APU system reliability. Some systems use cooling fans to accomplish this; however, cooling fans increase costs, weight, and contribute to the noise levels around the APU. Exhaust eductors are increasingly being used in APU gas turbine applications to cool, for example, APU compartment air, and/or gearbox and generator oil. Exhaust eductors generally include a primary exhaust nozzle configured to transport an active flow stream of the APU. A mixing duct at least partially surrounds the primary exhaust nozzle and transports a passive flow stream that is entrained by mixing with the active flow stream from the primary exhaust nozzle. The entrained passive flow stream flows through and cools the APU.
The mixed passive and active flow streams can become turbulent in the mixing duct and create eddies that recirculate back into the APU compartment. This results in inefficiencies, and can result in a failure of the exhaust eductor system to cool the APU. Temperatures at the inlet of the mixing duct can reach temperatures as high as 400-700° F. This is particularly a problem when the active flow stream is at an angle relative to the mixing duct.
Accordingly, there is a need for an exhaust eductor system that prevents recirculation of the mixed active and passive flow streams in the mixing duct. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background